The invention relates to an adaptive array sensor for a pixel-parallel detection of modulated signals with a high dynamic range, to an on-chip electrical circuit therefor, and to a method for adaptively detecting a modulated signal. The signals to be detected are, e.g., coherent or heterodyne analog intensity modulated optical signals. The invention is especially well adapted to the fields of optical measurement technique where it is required to detect the amplitude of a small light intensity modulation superimposed on a high slow-moving background. This is usually the case in interferometric methods, for example single or multiple wavelength interferometry, and especially optical low-coherence tomography (OLCT).
An example of multiple wavelength interferometry is disclosed in E. Zimmermann, Y. Salvadxc3xa9, and R. Dxc3xa4ndliker, xe2x80x9cStabilized three-wavelength source calibrated by electronic means for high-accuracy absolute distance measurementxe2x80x9d, Opt. Lett. 21 (7), 531 (1996). Principle and examples of OLCT are given in, e.g., E. A. Swanson et al., xe2x80x9cHigh-Speed optical coherence domain reflectometryxe2x80x9d, Optics Letters 17 (2), pp 151-153, 1992 or in G. J. Tearney, B. E. Bouma, S. A Boppart, B. Golubovic, E. A Swanson, J. G. Fujimoto, xe2x80x9cRapid acquisition of in vivo biological images by use of optical coherence tomographyxe2x80x9d, Opt. Lett., Vol. 21 (17), 1408 (1996).
The related art concerns array sensors which perform detection sampling and demodulation of a periodic signal of known frequency, using a CCD (e.g., FR-2 664 048) or a lock-in CCD (e.g., DE-44 40 613). An off-chip electrical or software signal conditioning is necessary to obtain the information on the amplitude modulation. The disadvantages of such techniques are the following:
It is necessary to know the frequency of the modulation signal exactly.
It is necessary to perform a substantial amount of signal post-processing to extract the envelope of the intensity modulation. This increases also the overall measurement time and system complexity.
The use of standard CCD devices limits the carrier frequency of the optical signal to detect due to the frame rate of the sensor. Indeed, it is necessary to acquire more than two samples per period of the carrier frequency to obtain the amplitude of the modulation. This means that the frame rate has to be more than two times faster than the carrier frequency.
The background of light limits the dynamic range of standard CCD image sensors.
This invention overcomes all these disadvantages by providing pixel circuitry with an adaptive feedback loop.
An AC amplifier circuit with a feedback loop has already been described in U.S. Pat. No. 5,376,813. It concerns an adaptive photoreceptor semiconductor circuit for long-time-constant continuous adaptation. The principle is based on a source follower receptor with feedback loop for adaptation of slow changes of background light. This circuit was developed to be used for time-domain image processing, for example, motion computation. It is useful in systems that care about the contrast change in an image, and not the absolute intensities. However, the use of the source follower principle for the detection stage has the disadvantage of placing a low impedance in series with the photodiode. This means that the AC transimpedance gain is small.
The invention aims at providing an on-chip electrical circuit for the adaptive detection of modulated signals which does not suffer from the above disadvantages. The invention further aims at providing a one-dimensional or a two-dimensional adaptive array sensor comprising a plurality of such circuits.
The electrical circuit according to the invention comprises sensor means for transducing an input signal into an electrical signal, amplifier means with an input and an output, and feedback means with an input and an output, the input being connected to the output of the amplifier means. The feedback means include frequency filter means for passing electrical signals modulated with frequencies within a selected frequency range and for blocking electrical signals modulated with frequencies out of the selected frequency range. The circuit further comprises a current source with an input and an output, its output current being controlled by an electrical input signal, the input being connected to the output of the feedback means and the output being connected in series with the sensor means in a common node. The common node of the current source and the sensor means is connected to the input of the amplifier means.
The AC transimpedance gain of the electrical circuit according to the invention is larger than with a source follower. The electrical circuit according to the invention is also more adapted for high gain AC amplification, and the change of the AC gain as a function of the DC level of the input signal is smaller than with the circuit according to U.S. Pat. No. 5,376,813. The presented invention preferably concerns also a whole circuit for amplitude demodulation, which is not the case for the circuit described in U.S. Pat. No. 5,376,813. No additional signal processing is necessary; indeed, the wanted information, i.e., the amplitude of the modulation, is directly and simultaneously readable at any desired time at the output of the circuit.
Intrinsically, the frequency range of the circuit according to the invention is not limited; typical modulation frequencies to be detected are between 1 kHz and 1 GHz.
The method for adaptively detecting a modulated signal according to the invention comprises the steps of transducing the signal into a first electrical signal, amplifying a second electrical signal including the first electrical signal, and using a part of the amplified electrical signal for a feedback. The part of the amplified electrical signal used for a feedback is frequency filtered so that electrical signals modulated with frequencies within a selected frequency range are passed and electrical signals modulated with frequencies out of the selected frequency range are blocked. The filtered electrical signals control a third electrical signal, and before amplification, the first electrical signal is combined with the third electrical signal, thus yielding the second electrical signal.
The invention also relates to a (one- or two-dimensional) array sensor which contains a multitude of similar or identical circuits according to the invention, with parallel outputs. This sensor with xe2x80x9csmart pixelsxe2x80x9d allows one to perform coherent or heterodyne detection of modulated signals simultaneously, in parallel for all pixels. The array sensor has a high dynamic range and, in the case of an optical sensor, a performance close to the shot noise limit of the light. The feedback loop of each circuit compensates for any imperfection or discrepancy on the array-sensor chip; such imperfections are inevitable and deteriorate the performance of a sensor according to the state of the art. The array sensor according to the invention may use the analog heterodyne or coherent detection method. In this case, xe2x80x9canalogxe2x80x9d means without sampling of the carrier frequency. In comparison with the direct-detection method, the heterodyne detection has the following advantages:
it is insensitive to unwanted background light with which the local oscillator does not mix;
heterodyning is one of the few ways of attaining photon-noise-limited detection in the infrared, where background noise is so prevalent.
The array sensor and the electrical circuit according to the invention can be used for any input signals such as electromagnetic, ultrasonic or chemical signals. However, in the following, the invention is discussed for the example of an optical signal.